It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said to Sayyiduna Bilal (may Allah be pleased with him): "O Bilal! Leave between your adhan and your iqamah a gap as long as it takes one who is eating to finish his food, one who is drinking to finish his drink, and one who needs to relieve himself to finish his need; and do not stand up until you see me."
There is some question about its chain (fi isnadihi nazar).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2282