Prophet Muhammad (pbuh) used to say the following invocations after finishing his obligatory prayers:

In Arabic, “Astaghfirullah” three times, and once “Allahumma anta al-salam wa minka al-salam tabarakta ya dhal-jalali wa’l-ikram.”

Which means, “O Allah! I seek Your forgiveness. O Allah! You are the Perfection and You are the One Who grants perfection; Blessed are You, O Owner of Majesty and Honor.”

Thauban reported: When the Messenger of Allah (ﷺ) finished his prayer. He begged forgiveness three times and said: “O Allah! Thou art Peace, and peace comes from Thee; Blessed art Thou, O Possessor of Glory and Honour.” Walid reported: I said to Auza’i: How is the seeking of forgiveness? He replied: You should say: “I beg forgiveness from Allah, I beg forgiveness from Allah.” – Sahih Muslim, Book 4, Hadith 1226.

Then, he (PBUH) used to remember Allah with the invocations reported to have been observed by him.

In Arabic, “La ilaha ill-Allah wahdahu la shareeka lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadeer. La hawla wa la quwwata illa Billah wa la na’budu illa iyyah. Lahu’l-ni’mah wa lahu’l-fadl wa lahu’l-thana al-hasan. La ilaha ill-Allah mukhliseena lahu’l-deena wa law kariha’l-kafiroon. Allahumma la mani’a lima a’tayta wa la mu’tiya lima mana’ta wa la yanfa’u dha’l-jaddi minka al-jadd.”

Which means, “There is no god but Allah Alone, with no partner or associate His is the sovereignty and to Him be praise, and He is Able to do all things. There is no power and no strength except with Allah, and we worship none but Him.  From Him (alone) come all blessings and favours, and all good praise is due to Him. There is no god but Allah and we make our worship purely for Him (alone) however much the disbelievers may hate that. O Allah, none can withhold what You give and none can give what You withhold, and no wealth or majesty can benefit anyone for from You is all wealth and majesty.”

Then after that he (pbuh) used to say,

In Arabic, “SubhanAllah”, “Alhamdulillah”, “Allahu Akbar” each thirty-three times, and once “La ilaha ill-Allah wahdahu la shareeka lah, lahu’l-mulku wa lahu’l-hamd, wa huwa ‘ala kulli shayin qadeer.”

Which means, “Glory be to Allah, praise be to Allah and Allah is most great. There is no god but Allah Alone, with no partner or associate His is the sovereignty and to Him be praise, and He is Able to do all things.”

Narrated Abu Huraira: Some poor people came to the Prophet (ﷺ) and said, “The wealthy people will get higher grades and will have permanent enjoyment and they pray like us and fast as we do. They have more money by which they perform the Hajj, and `Umra; fight and struggle in Allah’s Cause and give in charity.” The Prophet said, “Shall I not tell you a thing upon which if you acted you would catch up with those who have surpassed you? Nobody would overtake you and you would be better than the people amongst whom you live except those who would do the same. Say “Subhana l-lah”, “Al hamdu li l-lah” and “Allahu Akbar” thirty-three times each after every (compulsory) prayer.” We differed and some of us said that we should say, “Subhan-al-lah” thirty-three times and “Al hamdu li l-lah” thirty-three times and “Allahu Akbar” thirty-four times. I went to the Prophet (ﷺ) who said, “Say, “Subhan-al-lah” and “Al hamdu li l-lah” and “Allahu Akbar” all thirty-three times.” – Sahih Al Bukhari, Vol. 1, Book 12, Hadith 804.

Yahya related to me from Malik from Abu Ubayd, the mawla of Sulayman ibn Abd al-Malik, from Ata ibn Yazid al-Laythi that Abu Hurayra said, “Whoever says ‘Glory be to Allah’ (Subhana’llah) thirty-three times and ‘Allah is Greater’ (Allahu akbar) thirty-three times and ‘Praise be to Allah’ (al-hamdu lillah) thirty-three times, and seals the hundred with ‘There is no god but Allah, alone without any partner. The Kingdom and praise belong to Him and He has power over everything’ (La ilaha illa’llah, wahdahu la sharika lah, lahu’l mulku wa lahu’l hamd, wa huwa ala kulli shay’in qadir) after every prayer will have his wrong actions forgiven him even if they are abundant as the foam on the sea.” – Muwatta Malik, Book 15, Hadith 22.