Al-Qasim bin Abd al-Rahman narrates from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever comes in a state of ablution to offer an obligatory prayer, his reward is like the reward of one performing Hajj in ihram; whoever comes to offer the voluntary Duha prayer, with nothing but it making him set out, his reward is like the reward of one performing Umrah; and a prayer offered after a prayer, with no idle talk between them, is recorded in Illiyyin."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4910