It is narrated from Abd al-Rahman bin Abi Layla (may Allah have mercy on him) that Bilal (may Allah be pleased with him) was ordered to say 'As-salatu khayrun min an-nawm' (Prayer is better than sleep) in the morning prayer, and he was not ordered to say 'As-salatu khayrun min an-nawm' (Prayer is better than sleep) in any other.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1988