Positions in Prayer
4-9-2002 | IslamWeb
Question:
1) How are we to make a straight line for prayer with our toes or our heels aligned? Which way is Sunnah?
2) How should we sit in the second and last Raka'h?
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
Imam Abu Daawood reported in his Sunan from Anas 
that the Prophet faced them and said: “Straighten your rows (thrice); by Allah you have to straighten your rows, otherwise Allah will turn your hearts against one another.” The narrator (Anas) said: “I saw that everyone joined his shoulder to the shoulder of his companion, his knee to the knee of his companion, and his ankle to the ankle of his companion.” [Sheikh al-Albani graded it as Saheeh –authentic]
Ankle means the joint at the bottom of the leg where the foot begins and that could be joined to another ankle. The above explanation of Anas proves the practice of the Companions at the time of the Prophet . Therefore, it is an authentic proof to rely on. They did so only to be very careful in straightening and joining the rows.
The characteristic mentioned above is supported by other authentic pieces of evidence; thus, one should act in accordance to them.
For your second question, know that the Muslim scholars have different opinions concerning the way of sitting in Tashahhud. Probably the most correct opinion is that sitting on the entire posterior in the last Tashahhud of three or four Rak'ahs prayer is recommended. But in prayer of two Rak'ahs, whether it is obligatory or voluntary, or in the first Tashahhud, one should sit upon his left leg and keep his right foot upright. The evidence for the above statement is the Hadeeth of Abu Humayd as- Saa'idi who said in the gathering of some of the Companions: “I remember the prayer of the Prophet better than anyone of you. Then, he described the whole prayer and said: “… On sitting in the second Rak'ah, he sat on his left foot and propped up the right one; and in the last Rak'ah, he pushed his left foot forward and kept the other foot propped up and sat over the buttocks.” [Al-Bukhari]
There is a narration in all the books of Sunan, save the Sunan of an-Nasa'i, that reads: “In the last Rak'ah of the prayer, he would spread his left foot out under him and sat on his buttocks before finishing the prayer with Salaam.”
The above Hadeeth is an evidence for sitting on his buttocks in the last Rak'ah of the prayer of four Rak'ahs. The same ruling applies to the prayer of three Rak'ahs like the Maghrib prayer. Sitting on the left foot in prayer other than the above is confirmed from the above-mentioned Hadeeth and also from the Hadeeth of Waa-il ibn Hujr that he observed the Prophet's prayer and said: “He (the Prophet) prostrated and then sat down on his left foot.” [Ahmad, Abu Daawood, at-Tirmithi and an-Nasaa'i]
For more benefit, please refer to Fataawa 86287 and 88447.
Allah knows best.