صحيح الادب المفرد
Sahih al-Adab al-Mufrad
Part Two: Ahadith 250 to 499
باب ليعزم الدعاء؛ فإن الله لا مكره له
Chapter: Be firm in supplication; Allah cannot be forced
Hadith Number: 475
475/607 عن ابي هريرة، ان رسول الله صلى الله عليه وسلم قال:" إذا دعا احدكم، فلا يقول: إن شئت، وليعزم المسالة، وليعظم الرغبة فإن الله لا يعظم عليه شيء اعطاه".
It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When any one of you makes supplication, he should not say, '(O Allah) if You will, then grant it,' but he should supplicate with determination and increase his request, for nothing is too great for Allah to give."
تخریج الحدیث: (صحيح)