Incomplete front rows and Not straightening the rows during congregational prayers.

Narrated Jabir bin Samura (Radia-Allaahu ‘anhu) that the Prophet (Salla-Allaahu alaihi wa sallam) once came to us and said: “Why do you not make your lines (in prayer) as do the angels before their Lord?” We submitted: “O Messenger of Allah! How do the angels stand before their Lord.” He (Salla-Allaahu alaihi wa sallam) answered: “They complete the front rows first and stand close to each other.” [Muslim]

Narrated Anas (Radia-Allaahu ‘anhu): “We used to stand closely shoulder to shoulder and foot to foot.” [Al-Bukhari]

The Prophet (Salla-Allaahu alaihi wa sallam) said: “Whoever closes a gap (during congregational prayer), Allah will raise him with it one degree and build a house for him in Paradise.” [At-Tabarani and Sahih at-Targhib wat-Tarhib no. 505]

To pray in discontinuous lines:

Narrated Qurrah bin Iyas: “We were prohibited to stand in lines between the columns, at the time of the Prophet (Salla-Allaahu alaihi wa sallam). We were pushed away from them.” [Ibn Majah and authenticated by al-Albani in Sahih Ibn Majah no 1002]

The Prophet (Salla-Allaahu alaihi wa sallam) also said: “Straighten your rows or Allah will alter your faces.” [Al-Bukhari]

Narrated Ibn Umar رضي الله عنهما: Allah’s Messenger (Salla-Allaahu alaihi wa sallam) said, “Straighten the rows, stand shoulder to shoulder, close the gaps, be accommodating to your brethren, and do not leave gaps for Satan. Whoever joins up a row, he will be joined to Allah (i.e. to the mercy of Allah); and whoever cuts off a row, he will be cut off from Allah (i.e. from His Mercy).” [Abu Dawoud, Sahih at-Targhib wat-Tarhib no. 495]

Narrated Anas (Radia-Allaahu ‘anhu): Allah’s Messenger (Salla-Allaahu alaihi wa sallam) said: “Stand close together in your rows, keep near to one another, and put your necks in line (i.e. stand shoulder to shoulder), for by Him in Whose Hands my soul is, I see the Satan entering through the opening in the row like al-Hadhaf (i.e. a type of small black sheep found in Yemen.” [Abu Dawoud, Sahih at-Targhib wat-Tarhib no. 494]

Narrated al-Baraa’ bin Azib (Radia-Allaahu ‘anhu): Allah’s Messenger (Salla-Allaahu alaihi wa sallam) used to pass between the rows from one end to the other, touching our chest and shoulders (i.e., arranging the rows in line) and saying: “Do not be out of line; otherwise your hearts will be in disagreement.” [Abu-Dawoud, Sahih at-Targhib wat-Tarhib no. 513]