I remember once reading about an old fire worshipper of ninety years of age in Tadhkiratul-Awliya.’ It so happened that in a period of extended showers, the man was feeding seeds to the birds on the roof of his house. A noble man approached him and said: ‘Old man, what are you doing?’ He responded: ‘Brother, the rain has been falling incessantly for six or seven days now and so I am feeding the birds some grain.’
‘You do so in vain,’ said the noble man, ‘You are a disbeliever, what reward can you attain for such action?’ The old man responded, ‘I will surely receive my reward.’
The noble man relates that he went for Hajj and saw the same old man circuiting the Ka‘bah from a distance. The noble man was astonished to see this and relates that when he approached him, the old man was first to say: ‘Did my feeding seeds to the birds go in vain or have I received my reward?’
Now one ought to reflect that Allah the Almighty did not even waste the good deed of a disbeliever, then would He let the virtuous deed of a Muslim go in vain?
(Malfuzat, Vol. 1, p. 74)