The Masjid whose foundation was founded upon piety is the Masjid of the Prophet saws in Al-Madinah

Abu Salama b. Abd al-Rabman reported: 'Abd al-Rabman b. Abu Sa'id al-Khudri (Allah be pleased with him) happened to pass by me and I said to him. How did you hear your father making mention of the mosque founded on Piety? He said: My father said: I went to Messenger of Allah ﷺ as he was in the house of one of his wives, and said: Messenger of Allah, which of the two mosques is founded on piety? Thereupon he took a handful of pebbles and threw them on the ground and then said: This is the very mosque of yours (mosque at Medina). He (the narrator) said: I bear witness that I heard your father making mention of it.

حَدَّثَنِي مُحَمَّدُ بْنُ حَاتِمٍ، حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، عَنْ حُمَيْدٍ الْخَرَّاطِ، قَالَ سَمِعْتُ أَبَا سَلَمَةَ بْنَ عَبْدِ الرَّحْمَنِ، قَالَ مَرَّ بِي عَبْدُ الرَّحْمَنِ بْنُ أَبِي سَعِيدٍ الْخُدْرِيِّ قَالَ قُلْتُ لَهُ كَيْفَ سَمِعْتَ أَبَاكَ يَذْكُرُ فِي الْمَسْجِدِ الَّذِي أُسِّسَ عَلَى التَّقْوَى قَالَ قَالَ أَبِي دَخَلْتُ عَلَى رَسُولِ اللَّهِ ﷺ فِي بَيْتِ بَعْضِ نِسَائِهِ فَقُلْتُ يَا رَسُولَ اللَّهِ أَىُّ الْمَسْجِدَيْنِ الَّذِي أُسِّسَ عَلَى التَّقْوَى قَالَ فَأَخَذَ كَفًّا مِنْ حَصْبَاءَ فَضَرَبَ بِهِ الأَرْضَ ثُمَّ قَالَ " هُوَ مَسْجِدُكُمْ هَذَا " . - لِمَسْجِدِ الْمَدِينَةِ - قَالَ فَقُلْتُ أَشْهَدُ أَنِّي سَمِعْتُ أَبَاكَ هَكَذَا يَذْكُرُهُ .