Muwatta Malik 153

وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ الْمُؤَذِّنَ، جَاءَ إِلَى عُمَرَ بْنِ الْخَطَّابِ يُؤْذِنُهُ لِصَلاَةِ الصُّبْحِ فَوَجَدَهُ نَائِمًا فَقَالَ الصَّلاَةُ خَيْرٌ مِنَ النَّوْمِ ‏.‏ فَأَمَرَهُ عُمَرُ أَنْ يَجْعَلَهَا فِي نِدَاءِ الصُّبْحِ ‏.‏

Yahya related to me from Malik that he had heard that the muadhdhin came to Umar ibn al-Khattab to call him to the subh prayer and found him sleeping, so he said, "Prayer is better than sleep," and Umar ordered him to put that in the adhan for subh

Reference

Muwatta Malik 153

In-book

Book 3, Hadith 8