Abd al-Rahman (bin Abi Layla) narrates from Sayyiduna Bilal (may Allah be pleased with him); Bilal (may Allah be pleased with him) said: "The Messenger of Allah (peace and blessings be upon him) ordered me to make the tathwib in (the adhan of) Fajr (that is, to say 'Prayer is better than sleep'), and forbade me from making the tathwib in Isha."
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1088