Tafsīr al-ʿAyyāshi: From Moḥammed bin Sinān, from Abī al-Ḥassan, Mūsa bin Jaʿfar:

From his father [al-Ṣādiq] عليهما السلام, who said: He said to Abū Ḥanīfa: Which sūrah (chapter) is it that it’s beginning is praise, its middle is sincerity, and it’s ending is supplication?
Abu Hanifa remained perplexed, and then said: I don’t know.
Then Abū ʿAbd Allāh عليه السلام said: The sūrah in which it’s beginning is praise, it’s middle is sincerity, and it’s ending is supplication – is sūrah al-Ḥamd (i.e. al-Fātiḥa).1

1. The Imām عليه السلام is highlighting the chapter’s (sūrah) has special significance, with stated that it’s beginning is praise – because of the verse “All Praise is due to Allāh.” According to the Arabic linguists, the alif and lām (ال) prefix on the word الحمد (al-Ḥamd, all Praise) signifies الاستغراق  – meaning it is inclusive of all praise, because everything that is a blessing is from Allāh.
There’s a narration that says Allāh revealed it this way because mankind would not be able to praise Him with the praise which is due to Him, so by saying “al-Ḥamdulillah”, one is saying all praise – in absolute – is for Allāh alone.
Then the Imām said that it’s middle is sincerity, due to the verse “You alone do we worship and you alone do we seek help and aid.” This means we show our sincerity and faith by relying upon Allāh and devoting ourselves to Him whole heartily. Due to the structure of words in this sentence, the language demands that we worship only Allāh, and only from Allāh do we help and aid.
Then the Imām continues and says that it’s ending is supplication – “Guide us to the straight path – The path of those upon whom You have bestowed favor, not of those who have evoked [Your] anger or of those who are astray.
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عن محمد بن سنان ، عن أبي الحسن موسى بن جعفر ، عن أبيه ( عليهما السلام ) ، قال : قال لأبي حنيفة : ما سورة أولها تحميد ، وأوسطها إخلاص ، وآخرها دعاء ؟ فبقي متحيرا ، ثم قال : لا أدري . فقال أبو عبدالله ( عليه السلام ) : السورة التي أولها تحميد ، وأوسطها إخلاص ، وآخرها دعاء ، سورة الحمد