It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whichever Muslim performs ablution (wudu) well and then offers the obligatory prayer, it is for him like a Hajj; and if he offers a voluntary prayer, it is for him like an Umrah."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7913