Arabic text
عن عبد الله بن عمرو رضي الله عنهما أن النبي صلى الله عليه وسلم قال: «الرَّاحِمُونَ يَرْحَمُهمُ الرَّحمنُ، ارحَمُوا أهلَ الأرضِ يَرْحْمْكُم مَن في السّماء».
Translation
‘Abdullāh ibn ‘Amr (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) said: "Those who are merciful are shown mercy by the Most Merciful. Be merciful to the inhabitants of earth and the One in the heaven will be merciful to you."
Explanation
The Prophet (may Allah's peace and blessings be upon him) points out that those who show mercy to others are shown mercy by the Most Merciful with His mercy, which encompasses all things - a fitting recompense. Then, he (may Allah's peace and blessings be upon him) enjoined mercy towards all those on earth, including human beings, animals, birds, and other kinds of creatures. As recompense for that, Allah, from above His heavens, will show mercy to you.
Source: HadeethEnc.com