Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.011.

‘Abdullah Ibn Mughaffal reports that the Prophet said: “Between every azhan and iqamah there is a prayer. Between every azhan and iqamah there is a prayer.” And, after saying it a third time, he said: “For whoever wishes [to pray it].” This is related by the group. Ibn Hibban records from Ibn az-Zubair that the Prophet said: “There exists no obligatory prayer without there being, immediately preceding it, two rak’at.”