It is narrated from Saeed ibn al-Musayyib who said: One year famine occurred and people prayed for rain.

I looked and saw a black slave went on top of a hill and separated himself from other people. I went after him and saw his lips are moving and his Dua still not finished when cloud appeared in sky. When the slave saw the cloud, he praised Allah (SWT) and moved from there and we were all caught in the rain so much that we thought it will destroy us. I followed that slave and saw him went inside the house of Imam Sajjad (AS). I went to see Imam (AS) and said: You have a black slave, please sell him to me. The slave asked Saeed: Why do you want to separate me from my Imam (AS)? Saeed replied: Because of what I saw when your Dua was accepted for rain. That slave raised his hands towards the sky and said: O’ God, this was a secret between us, now that Saeed is aware of it, take me to Yourself; and he died [1].

[1] Montahul Amaal, Sheikh Abbas Qomi, Vol. 2, Page 38.

Source: en.shafaqna.com