Mishkat al-Masabih 1320
Abu Sa'id said:
God’s Messenger used to pray in the forenoon so often that we felt he would never stop doing so; then he would leave off doing so for so long that we felt he would never resume prayer at that time. 

Tirmidhi transmitted it.
وَعَنْ أَبِي سَعِيدٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُصَلِّي الضُّحَى حَتَّى نَقُولَ: لَا يَدَعُهَا وَيَدَعُهَا حَتَّى نَقُولَ: لَا يُصليهَا. رَوَاهُ التِّرْمِذِيّ