Question Summary:
Is the hadith, “Sleep of the fasting person is worship” authentic?

Question Detail:

I heard a hadeeth from the Prophet (peace and blessings of Allah be upon him): “The sleep of the fasting person is worship.” Is this hadeeth saheeh (authentic)?

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
The hadīth you refer to has been narrated by Imām al-Bayhaqī in his Shu'āb al-Īmān as follows:
قَالَ رَسُولُ اللهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: "نَوْمُ الصَّائِمِ عِبَادَةٌ، وَصَمْتُهُ تَسْبِيحٌ، وَدُعَاؤُهُ مُسْتَجَابٌ، وَعَمَلُهُ مُضَاعَفٌ" 
The Messenger of Allah (sallallahu 'alayhi wa sallam) said: "The sleep of a fasting person is worship, his silence is tasbīh, his duā is accepted, and (the reward of) his actions are multiplied."[1]
After narrating this hadīth, Imām al-Bayhaqī mentions that three narrators in the chain of this hadīth are weak.[2] 'Allāmah al-Īrāqī also classifies this hadīth as weak in his Takhrīj Ahādīth Ihyā` 'Ulūm al-Dīn[3]. 'Allāmah al-Munāwī has also stated the same in his Fayd al-Qadīr[4].
Although this hadīth may not be classified as authentic, we may still apply the general meaning of this hadīth for any person who sleeps with the intention of gaining strength for making more 'ibādah. Consider the conversation between Mu'ādh bin Jabal and Abū Mūsā al-Ash'arī (radiyallahu 'anhumā) when they met in Yemen:
قَالَ: يَا عَبْدَ اللَّهِ، كَيْفَ تَقْرَأُ القُرْآنَ؟ قَالَ: أَتَفَوَّقُهُ تَفَوُّقًا، قَالَ: فَكَيْفَ تَقْرَأُ أَنْتَ يَا مُعَاذُ؟ قَالَ: أَنَامُ أَوَّلَ اللَّيْلِ، فَأَقُومُ وَقَدْ قَضَيْتُ جُزْئِي مِنَ النَّوْمِ، فَأَقْرَأُ مَا كَتَبَ اللَّهُ لِي، فَأَحْتَسِبُ نَوْمَتِي كَمَا أَحْتَسِبُ قَوْمَتِي
Mu'ādh (radiyallahu 'anh) said: "O `Abdullah (bin Qais)! [referring to Abū Mūsā (radiyallahu 'anh)]: How do you recite the Qur'an?" Abū Mūsā (radiyallahu 'anh) said, "I recite the Qur'an regularly at intervals. How do you recite it O Mu'ādh?" Mu'ādh (radiyallahu 'anh) said, "I sleep in the first part of the night and then get up after having slept for the time devoted for my sleep and then recite as much as Allah has written for me. So I seek Allah's Reward for both my sleep as well as my prayer (at night)."[5]
Commenting on the last portion of this hadīth, Hāfidh ibn Hajar says in his Fath al-Bārī:
مَعْنَاهُ أَنَّهُ يَطْلُبُ الثَّوَابَ فِي الرَّاحَةِ كَمَا يَطْلُبُهُ فِي التَّعَبِ لِأَنَّ الرَّاحَةَ إِذَا قُصِدَ بِهَا الْإِعَانَةُ عَلَى الْعِبَادَةِ حَصَّلَتِ الثَّوَابَ 
This means that he (Mu'ādh radiyallahu 'anh) would seek reward through resting as he would seek reward through tiring himself out (in 'ibādah) because when one rests and makes an intention to gain strength for 'ibādah through it, he gains reward (through this intention of his as well).[6]
From this we can see that even though we are unable to classify this hadīth as authentic, the general message of the hadīth still holds that the sleep of a fasting person is an act of worship on the condition that he makes the intention of gaining strength to fast through his sleep.
And Allah Ta’āla Knows Best
Bilal Mohammad
Student Darul Iftaa
USA
Checked and Approved by,
Mufti Ebrahim Desai.
www.daruliftaa.net