Imaam Ahmad used to mention the virtues of his Shaykh, Imaam Ash-Shaafi’ee to his wife. He would mention to her the extensive and superb knowledge of Imaam Ash-Shaafi’ee as well as his taqwah (i.e. piety). Imaam Ahmad invited Imam Ash Shafi’ee to come visit him at his home. When the time for dinner arrived, Imam Ash-Shaafi’ee ate until he was full and went to the guest room for some sleep.

The wife of Imam Ahmad said to him:

“O Ahmad! Is this the same Shaafi’ee that you used to tell me so much about?“

Imaam Ahmad said: “Indeed he is!“

So she said:

“I noticed three things about him worthy of criticism! Firstly, when we served him the food, he ate plenty of it! Secondly, when he went into the guest room, he went to sleep without standing up to perform the night prayer (i.e. tahujjud)! Lastly, when he prayed Salaat ul Fajr with us, he did not perform ablution (i.e. wudhoo')?!”

So Imaam Ahmad went to Imaam Ash-Shaafi’ee to ask him about these three matters his wife had noticed. Imam Ash-Shaafi’ee said to him, (the way a father would explain to his son):

“O Ahmad! I ate plenty because I know for certain that your food is haalal (i.e. islamically permissible ) and you are a generous man! And the food that comes from a generous man is a cure, while the food that comes from a stingy man is a disease! However, I didn’t eat plenty of food in order to achieve my fill; rather I ate plenty of your food in order to derive the cure from it!

As for the reason I didn’t stand up at night to perform the night prayer, when I laid my head down to get some sleep, I looked and all I could see was the Book of Allah and the Sunnah of His Prophet (sallallahu 'alaihi wa sallam) right in front of my face! So I deducted 72 fiqh related benefits from them for the Muslims to benefit from. Thus there was no opportunity for me to stand and perform the night prayer!

As for the reason why I prayed Salaat ul Fajr without performing ablution, by Allah! My eyes never tasted the sweetness of sleep in order for me to renew my wudhoo'. I was awake the whole night, so I prayed Salat ul Fajr with the same wudhu I had for Salaat ul Isha."

Source: Collected by Al Hafidh Ibn Hajr in Fath ul Bari concisely and by Abu Nu’aim in Hilya tul Awliyaa’.