(a) It is narrated from Sayyiduna Bilal (may Allah be pleased with him) that he said: The Messenger of Allah (peace and blessings be upon him) ordered me: "That I should not perform tathwib (in any other adhan) besides Fajr."
(b) It is narrated from Suwayd bin Ghafalah (may Allah have mercy on him) that Sayyiduna Bilal (may Allah be pleased with him) did not perform tathwib except in Fajr; in his adhan he would say: 'Hayya 'ala al-falah, as-salatu khayrun min an-nawm' (Come to success; prayer is better than sleep).
This too is mursal, for Abd al-Rahman bin Abi Layla did not meet Bilal.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1989