Not graded
Riyad as-Salihin 1099
وعن عبد الله بن مغفل، رضي الله عنه، قال: قال رسول الله صلى الله عليه وسلم: "بين كل أذانين صلاة، بين كل أذانين صلاة، بين كل أذانين صلاة" وقال في الثالثة: "لمن شاء" ((متفق عليه)).
'Abdullah bin Mughaffal (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) said, "There is a Salat (prayer) between every Adhan and Iqamah; there is a Salat between every Adhan and Iqamah." (While saying the same for the) third time (he (ﷺ) added), "It is for him who desires (to perform it)."
[Al-Bukhari and Muslim].
Cited in Riyad as-Salihin (No. 1099).