In the book al-Khara’ij by ‘Allamah al-Rawandi it is related through Salman (al-Farsi) that,
“One day I was in the house of Sayyidah Zahra. I saw that she was seated and grinding barley with a mill. I also saw that the handle of the mill was soaked with blood (from her hands) while Husayn, who was an infant, was weeping in the room due to excessive hunger.
I said, ‘O daughter of the Prophet of Allah! Do not strain yourself much, Fidhdhah is present here and at your service.’ She replied, ‘The Prophet of Allah has recommended to me that one day I should do the work of the house and one day let Fidhdhah do it. Yesterday was Fidhdhah’s turn while today is my turn.’
I said, ‘I am your ransomed slave and am at your service, either let me grind the barley or look after Husayn.’
She replied, ‘It befits me more to look after Husayn, while you may grind the mill.’ I started grinding the mill when suddenly I heard the call to prayer. I went to the masjid and offered prayers behind the Prophet of Allah.
After the prayers I related what had transpired to Imam ‘Ali who started weeping. He made his way towards the house and then returned back smiling. The Prophet of Allah asked him the reason for his smiling and he replied, ‘I went towards Fatimah and saw that she was sleeping while Husayn was sleeping on her chest and I saw that the grind-mill was turning around on its own.’
The Prophet smiled and said, ‘O ‘Ali! Do you not know that Allah has appointed some angels to circulate the earth and serve Muhammad and his progeny, and this service of theirs will continue until the Day of Resurrection.’”
House of Sorrows Translation of Baytul AhzanShaikh Abbas Al – Qummi