There is nothing wrong with making du`a’ after Salah. There are a number of hadiths that confirm that the Prophet (peace and blessings be upon him) made du`a’ after Salah. For example, Sa`d Ibn Abi Waqqas (may Allah be pleased with him) used to recommend these five (statements) and say that the Prophet (peace and blessings be upon him) said so (and they are): “O Allah! I seek refuge in You from miserliness, and I seek refuge in You from cowardice, and I seek refuge in You from being brought back to geriatric old age, and I seek refuge in You from the afflictions of the world, and I seek refuge with You from the punishment of the grave.” (Al-Bukhari, Sahih, vol. 8, hadith no. 381)
In a response to the question, the late Sheikh Muhammad Mitwalli Ash-Sha`rawi, a well-known Imam and renowned Muslim scholar, states the following:
“In his supplication after Prayer, the Prophet (peace and blessings be upon him) used to say, ten times, especially after Fajr and Maghrib, “There is no god but Allah, the One and Only ! There is no partner with Him, to Him belongs the sovereignty and to Him is the praise. He has the power to cause death and give life. He has power over all things!”

He (peace and blessings be upon him) also used to say: “I seek Allah’s forgiveness; He is the All-Living, the Eternal (three times).” and then say, ‘O Allah, You are the Peace, and You are the source of peace. You are filled with good, O Sublime and Honorable One!”

Part of what the Prophet, peace and blessings be upon him, also used to say is: “O Allah, none can withhold what You have conferred, nor can one confer what You have withheld. A fortune does not benefit its owner against You”; and after that he would read the Verse of the Throne (Al-Baqarah: 255) and then utter certain statements which, if one were to utter or observe them at the end of every obligatory Prayer, one would not be dismayed. They are: glorifying Allah thirty-three times, praising Allah thirty-three times and extolling His greatness thirty-four times.”
Dr. Muzammil Siddiqi, former head of the Islamic Society of North America (ISNA) and member of the Fiqh Council of North America, adds:
“Making du`a’ (supplication) after each Salah is mustahhab (recommended) according to all schools of Fiqh. The Prophet (peace and blessings be upon him) used to make dhikr and du`a’ after each Prayer. The discussion is about making a collective du`a’, whether it is permissible to make a congregational du`a’ after the Prayer. Some scholars of Hadith consider it to be mustahhab while others say it should not be done. However, a big deal should not be made out of it. If a person doesn’t want to do it, he should leave after the Salah. This is a minor issue.”