From Reward of taking care of widows and orphans (Islam.ru, 2014):
A man once came to the Messenger of Allah, sallallaahu`alaihiwasallam, and complained that he feels hardness in his heart. The Messenger of Allah said, what translated means, "Would you like that your heart becomes soft and that you acquire what you need? Be merciful with the orphan, pat his head and feed him from what you eat. This will soften your heart, and enable you to get what you need." [At-Tabaraanee & As-Silsilah as-Saheehah]
A hadith having similar meaning but slight variation of wording and transmitted through a different chain, was recorded in Musnad Ahmad, and the chain was classed as Hassan by Ibn Hajar in Fath al-Bari (كتاب الدعوات):
ولأحمد من حديث أبي هريرة أن رجلا شكى إلى النبي صلى الله عليه وسلم قسوة قلبه فقال أطعم المسكين وامسح رأس اليتيم وسنده حسن
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