It is narrated from Sayyiduna Abdullah bin Mughaffal al-Muzani (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Between every two calls there is a prayer." He said these words three times, "for whoever wishes."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2280