Al-Baqarah: 248
"And their prophet said to them..."
The Ark (al-tābūt)
It is the chest of the Torah. When Moses (peace be upon him) would fight, he would place it in front of him; the souls of the Children of Israel would then find tranquility, and they would not flee.
"And the Sakīnah (tranquility)..."
It is stillness and peace of mind. It is also said that it was a figure inside the Ark made of chrysolite or ruby, having a head like a cat’s head, a tail like its tail, and two wings. It would moan, and the Ark would move toward the enemy while they followed it. When it settled, they would stand firm, find peace, and victory would descend. It is narrated from Ali (may Allah be pleased with him) that it had a face like a human face and a gentle breeze.
"And a remnant (baqiyyah)..."
It is the fragments of the Tablets, the staff of Moses, his garments, and a portion of the Torah. Allah Almighty had raised it after Moses (peace be upon him), then the angels brought it down, carrying it while the people watched. This was a sign of Allah’s choosing of Saul (Ṭālūt).
It is said that it remained with Moses and the prophets of the Children of Israel after him, and they would seek victory through it. When the Children of Israel corrupted their ways, the disbelievers overcame them and took it; it remained in the land of Goliath (Jālūt). When Allah willed to make Saul king, He afflicted them with a plague until five cities perished. They said, "This is because the Ark is among us." So they placed it upon two oxen, and the angels drove them to Saul.
It is said it was made of boxwood, gilded with gold, measuring about three cubits by two. Ubayy and Zayd ibn Thābit read it as al-tābūh (with a hā'), which is the dialect of the Anṣār.
If you ask: What is the morphological weight of tābūt?
I say: It must be either fa‘alūt or fā‘ūl. It cannot be fā‘ūl because that form is rare (like sals and qalaq), and because it is an unknown construction; one should not abandon the known for the unknown. Therefore, it is fa‘alūt from al-tawb (returning), because it is a container in which things are placed and deposited; what is taken out of it is always returned to it, and its owner returns to it for what he needs of its contents. As for those who read it with a hā' (tābūh), they consider it fā‘ūl, unless they consider the hā' a substitute for the tā'—since both are whispered (hams) letters and both are letters of augmentation; for this reason, it is substituted for the feminine tā'.
Abū al-Summāl read sakīnah with a fatḥah on the sīn and a shaddah (emphasis), which is strange. It was also read as yaḥmiluhu (with a yā').
If you ask: Who are the "family of Moses and the family of Aaron"?
I say: They are the prophets from the descendants of Jacob after them, for ‘Imrān was the son of Qāhith, son of Lāwī, son of Jacob; thus, the children of Jacob were their family.
It is also permissible that it refers to what Moses and Aaron left behind, and the word "family" (āl) is inserted to magnify their status.