Somebody told me that reciting the Holy Quran without observing the principles of Tajweed was not acceptable, rather it was 'gunah' as many Arabic words have altogether different meanings with slight change in pronunciation which may have been due to lack of knowledge of Tajweed.

Question
I have not learnt Tajweed. Somebody told me that reciting the Holy Quran without observing the principles of Tajweed was not acceptable, rather it was 'gunah' as many Arabic words have altogether different meanings with slight change in pronunciation which may have been due to lack of knowledge of Tajweed. I was also told that in such a situation even the Namaz (Salah) becomes doubtful as we have to recite Quranic verses in it. Please clarify giving some authentic references from the Holy Quran and Hadith.

Answer
Assalaam alaikum,
Please see the following previously answered question and the PFD download in the link below:

http://www.abouttajweed.com/q2060102.htm
http://www.abouttajweed.com/index2.php?option=com_docman&gid=1&task=doc_view&Itemid=33

Insha' Allah, this will give you the information you need. 
Wa assalaam.