I heard the Messenger of Allah ﷺ say, "If any of you suffers from an ailment, or his brother suffers from one, let him say, 'Our Lord Allah, who is in heaven, hallowed be Your name. Your command prevails in heaven and on earth. As Your mercy is in heaven, place Your mercy on earth. Forgive us our sins and our wrongdoings. You are the Lord of the righteous. Send down a mercy from Your mercy and a healing from Your healing upon this pain,' and he will be cured."
حدثنا يزيد بن خالد بن موهب الرملي حدثنا الليث عن زيادة بن محمد عن محمد بن كعب القرظي عن فضالة بن عبيد
سمعت رسول الله ﷺ يقول من اشتكى منكم شيئا أو اشتكاه أخ له فليقل ربنا الله الذي في السماء تقدس اسمك أمرك في السماء والأرض كما رحمتك في السماء فاجعل رحمتك في الأرض اغفر لنا حوبنا وخطايانا أنت رب الطيبين أنزل رحمة من رحمتك وشفاء من شفائك على هذا الوجع فيبرأ
[Its chain is daif] Nasai narrated it in Kubra, no. 10877, and in Amal al-Yawm wal-Layla, no. 1038, from the hadith of Layth bin Saad, through this chain. Ziyad bin Muhammad is rejected in his hadith (Taqrib). Hakim erred by including it in Mustadrak, 12/344, 4/218, 219, and Dhahabi refuted him.