Ibn Mas’ud reported: The Prophet, peace and blessings be upon him, said, “When one of you bows in prayer, let him say three times in his bowing: Glory be to my Lord, the Greatest. He will have completed his bowing and that is the least of it. Let him say three times in his prostration: Glory be to my Lord, the Most High. He will have completed his prostration and that is the least of it.”
Al-Tirmidhi said, “This was acted upon by the people of knowledge, that they preferred a man glorify Allah no less than three times in bowing and prostration.”
Source: Sunan al-Tirmidhī 261
Grade: Hasan li ghayrihi (fair due to external evidence) according to Al-Arna’ut
عَنْ ابْنِ مَسْعُودٍ أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ إِذَا رَكَعَ أَحَدُكُمْ فَقَالَ فِي رُكُوعِهِ سُبْحَانَ رَبِّيَ الْعَظِيمِ ثَلَاثَ مَرَّاتٍ فَقَدْ تَمَّ رُكُوعُهُ وَذَلِكَ أَدْنَاهُ وَإِذَا سَجَدَ فَقَالَ فِي سُجُودِهِ سُبْحَانَ رَبِّيَ الْأَعْلَى ثَلَاثَ مَرَّاتٍ فَقَدْ تَمَّ سُجُودُهُ وَذَلِكَ أَدْنَاهُ
قال الترمذي وَالْعَمَلُ عَلَى هَذَا عِنْدَ أَهْلِ الْعِلْمِ يَسْتَحِبُّونَ أَنْ لَا يَنْقُصَ الرَّجُلُ فِي الرُّكُوعِ وَالسُّجُودِ مِنْ ثَلَاثِ تَسْبِيحَاتٍ
261 سنن الترمذي صفة الصلاة باب ما جاء في التسبيح في الركوع والسجود
886 المحدث شعيب الأرناؤوط خلاصة حكم المحدث حسن لغيره في تخريج سنن أبي داود