It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both): I offered eight rak‘ahs and seven rak‘ahs (combined) with the Messenger of Allah (peace and blessings be upon him). The narrator says: I asked Ibn Abbas (may Allah be pleased with them both), “Why was this done?” So Sayyiduna Ibn Abbas (may Allah be pleased with them both) replied, “So that there may be ease for the Ummah.”
Hadith Referenceالسنن المأثورة / ما جاء في الجمع بين الصلاتين في المطر / 24
Hadith Takhrijصحيح مسلم، المساجد و مواضع الصلاة باب الجمع بين الصلاتين في الحضر، رقم: 705