The Prophet of Allah ﷺ said, "Between every two calls to prayer there is a prayer." He repeated this three times, adding the third time, "for whoever wishes."
حدثنا أبو بكر بن أبي شيبة حدثنا أبو أسامة ووكيع عن كهمس حدثنا عبد الله بن بريدة
قال نبي الله ﷺ بين كل أذانين صلاة قالها ثلاثا قال في الثالثة لمن شاء
Bukhari recorded it in the Book of Adhan, chapter "Between every two calls to prayer there is a prayer for whoever wishes," hadith 627, from the hadith of Kahmas, with this wording. Muslim recorded it in the Book of the Prayer of Travelers, chapter "Between every two calls to prayer there is a prayer," hadith 838, from the hadith of Abu Usama and Waki, with this wording.