- If you do a favor for someone, forget it. --- إذا اتخذتم عند رجل يداً فانسوها

 If you do a favor for someone, forget it. This was said by one of the wise men of Arabia to his sons. Abu Ubaid said: He meant so that you would not feel indebted to people in your hearts, nor mention it with your tongues. He said: You spoil with reminders what you have done to ease your affairs; a generous person does not boast when he gives.

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The closest equivalent in English is:

Do a good deed and forget it.

Do good and then forget it.

Also:

Never remind people of the favors you did for them.

And a well-known proverb, very close in spirit, is:

Cast your bread upon the waters.

That is, do good without expecting anything in return or being reminded of it.

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قال بعض حكماء العرب لبنيه. قال أبو عبيد: أراد حتى لا يقع في أنفسكم الطول على الناس بالقلوب ولا تذكروها بالألسنة. قال:
أفسدت بالمن ما أصلحت من يسر*****ليس الكريم إذا أسدى بـمـنـان 

 

إن الموصين بنو سهوان ---- Those who make the will are forgetful

 This is a proverb that has caused much confusion in its interpretation, and the correct understanding is what I have established after recounting their various opinions. Some said: "Only those who are forgetful and negligent need a will, but you do not need one because you are not forgetful." Others said that by "the sons of forgetfulness," he meant all people because everyone is forgetful. The more accurate interpretation is that those who make a will are overcome by forgetfulness, as if it were their very nature.

This proverb is used for someone who forgets to seek something they were commanded to do. "Sahwan" means forgetfulness, and it can also be an adjective, meaning "the sons of a forgetful man," referring to Adam, peace be upon him, when he was entrusted with a task but forgot. It is said, "a forgetful man," meaning that those who make wills are not infallible because they are the sons of Adam, peace be upon him

ِAi said

"To err is human."
➜ Making mistakes (and forgetting) is human nature.
"Humans are forgetful by nature." 

(Direct expression)
➜ Humans are forgetful by nature.
"Practice what you preach."
➜ Said of someone who gives an order but doesn't follow it (similar in meaning to advise and forget).
"Easier said than done."
➜ It's easier to advise or give advice, but putting it into practice is harder (partially related in meaning)..

 هذا مثل تخبط في تفسيره كثير من الناس والصواب ما أثبته بعد أن أحكي ما قالوا. قال بعضهم: إنما يحتاج إلى الوصية من يسهو ويغفل فأما أنت فغير محتاج إليها لأنك لا تسهو وقال بعضهم يريد بقوله بنو سهوان جميع الناس لأن كلهم يسهو والأصوب في معناه أن يقال إن الذين يوصون بالشيء يستولي عليهم. السهو حتى كأنه موكل بهم.
يضرب لمن يسهو عن طلب شيء أمر به. والسهوان السهو ويجوز أن يكون صفة أي بنو رجل سهوان وهو آدم عليه السلام حين عهد إليه فسها ونسي يقال رجل سهوان وساه أي إن الذين يوصون لا بدع أن يسهوا لأنهم بنو آدم عليه السلام.